1. Field of the Invention
This invention relates to an apparatus for measuring characteristics of a liquid. In particular, but not exclusively, the invention relates to an ultraviolet (UV) absorbency-based monitor for measuring the amount of organic pollution in a liquid, and is of application for on-line monitoring of water quality at locations such as industrial or water treatment plant effluent outlets, rivers or reservoirs.
2. Description of the Prior Art
A number of methods are available for determining the biological or organic pollution in water, involving the measurement of parameters such as BOD (biochemical oxygen demand), TOC (total organic carbon) or COD (chemical oxygen demand). The parameter that is usually of most interest is the BOD.
A number of on-line, continuous BOD monitors are available that operate by measuring the absorbency of ultraviolet (UV) light by a sample. These monitors are based on the principle that a particular substance will absorb light of a particular wavelength (or wavelengths), so that the reduction in intensity of light of that wavelength, when transmitted through a sample containing the substance, can be related to the concentration of that substance. Aromatic organic compounds, and other organic compounds that have conjugated double bonds, absorb light in the UV wavelength region and research has shown that absorbency measurements taken in that region, and in particular at 254 nm, can be related to BOD (and also to TOC and COD).
In one such monitor, the amount of organic matter in a liquid sample is determined by passing a liquid sample through a flow cell, passing UV and visible light through the sample and making measurements of the absorption of UV light by the sample and the absorption and scatter of visible light. The amount of organic matter in the sample is determined from the measured absorption of UV light, with the visible light measurements being used for compensation purposes.
One disadvantage of the known monitor is the tendency of the flow cell to become fouled by substances in the liquid sample, which can result in the windows for the UV and visible light monitors becoming obscured. This reduces the accuracy and efficiency of the monitor and can eventually prevent operation. It is necessary, therefore, to clean the flow cell at regular intervals, which is costly and time-consuming and can prevent automatic operation of the monitor.